1. Field of the Invention
The present invention relates to an image signal processing technique and, more particularly, to such technique for processing a decoded image signal obtained by decoding coded data by utilizing a two-dimensional inverse orthogonal transformation in which the coded data is obtained by coding an image signal by utilizing a two-dimensional orthogonal transformation which may, for example, be applicable in image transmission to a remote location using a transmitting medium having a limited transmission capacity or in digital recording and reproducing of an image on a video tape recorder or video disk recorder having a limited capacity.
2. Description of the Prior Art
In systems transmitting an image signal to a remote location, such as a video conference system, the amount of information transmitted may be reduced by efficiently coding significant information by utilizing the correlation between image signals. As a result, the transmission efficiency is enhanced. Similarly, in an apparatus for digitizing an image signal and for recording and reproducing such digitized signal on a video tape recorder or video disk recorder or the like, the amount of record information may be reduced by efficiently coding significant information by utilizing the correlation between digitized image signals, thereby enhancing the recording efficiency. Accordingly, in these situations, the transmission path and the recording medium are effectively utilized.
The above-mentioned coding may be performed by utilizing a block orthogonal transformation coding method, such as a discrete cosine transform (DCT). FIGS. 16 and 17 respectively illustrate an image coding apparatus 149 and an image decoding apparatus 159 which utilize a block orthogonal transformation coding method such as DCT and an inverse thereof. Each such apparatus will now be described.
In the image coding apparatus 149 of FIG. 16, an input image signal S140, which may be a scanning signal, is supplied to a blocking circuit 140. The blocking circuit 140 transforms the received signal into a blocked image signal S141 representing blocks of 8.times.8 pixels. An 8.times.8 block is utilized herein as a transformation unit. The blocked image signal S141 is supplied to an orthogonal transformation circuit 141 which transforms the received blocked image signal into a plurality of coefficients S142 for each coded block. Each such coefficient S142 is supplied to a quantization circuit 142 so as to be quantized with accuracy corresponding to the location of each coefficient within the coded block and supplied therefrom as a quantized coefficient S143.
It is to be noted that when the visual capabilities and the like of a human being are considered, a lower-order side coefficient including many components of a low region which is of significant importance in constructing an image is quantized with a relatively high degree of accuracy, whereas a higher-order side coefficient including many components of a high region which is of lesser importance in constructing an image is quantized with a relatively low degree of accuracy.
Each quantized coefficient S143 is supplied to a variable-length coding circuit 143, in which a code having a relatively short length is assigned to the quantized coefficient having a relatively high frequency of occurrence and a code having a relatively long length is assigned to the quantized coefficient having a relatively low frequency of occurrence. As a result, the variable-length coding circuit 143 produces coded data S144 wherein the sum or total of the code lengths, which is the information to be transmitted or recorded, is reduced.
The coded data S144 is supplied to a buffer memory 144 so as to smooth the information amount. An output signal S145 is supplied from the buffer memory 144 as an output of the image coding apparatus 149. The buffer memory 144 further supplies therefrom a quantized accuracy information signal S146, which represents the amount of storage of the buffer memory 144, as another output of the image coding apparatus 149. The quantized accuracy information signal S146 is also constantly fed back to the quantization circuit 142 and utilized therein for controlling the accuracy of the quantization, so that the speed of the output signal S145 may become constant.
The image decoding apparatus 159 of FIG. 17 generally performs inverse operations to those performed by the image coding apparatus 149 of FIG. 16 and, accordingly, only a cursory description thereof will be provided herein.
In the image decoding apparatus 159, coded data S150 and quantization accuracy information S156 are received from a transmitting medium or the like. The received coded data S150 is supplied through a buffer memory 150 to a variable-length decoding circuit 151 which, in turn, supplies quantized coefficients S151 therefrom. Each quantized coefficient S151 along with the received quantization accuracy information S156 are supplied to an inverse quantization circuit 152, wherein the quantized coefficients are inverse-quantized based on the quantization accuracy information and inverse-quantized coefficients S152 are produced. Each inverse-quantized coefficient S152 is supplied to an inverse orthogonal transformation circuit 153, which restores or transforms the received inverse-quantized coefficient to a blocked image signal S153 for each coded block. The blocked image signal S153 is supplied to a scanning signal circuit 154, whereupon the same is formed as a scanning signal and supplied therefrom as an output image signal S154 of the image decoding apparatus.
The above-described coding technique enables an image to be relatively easily restored with a high picture quality and with a high compression efficiency. Accordingly, such coding technique utilizing a block orthogonal transformation, such as DCT, is widely used. However, if the amount of coded data is reduced so as to enhance the compression efficiency, the quantization accuracy may not be sufficiently defined. As a result, a problem or drawback may arise wherein a relatively easily observable block-shaped distortion occurs. That is, in this situation, a block-shaped distortion occurs due to the insufficient accuracy of the quantization of the transformation coefficient(s).
To remove or minimize the above-described block distortion, the quantization accuracy may be enhanced. However, if the quantization accuracy is enhanced, then the compression efficiency will be reduced. As a result, the effective utilization of the respective medium, which has a limited data capacity, cannot be effected. As is to be appreciated, such effective utilization of the respective medium is a primary or fundamental objective. In an effort to solve this problem, a postprocessing technique which eliminates block distortion and enhances the picture quality by processing a restored image without reducing the compression efficiency has been developed. Such technique will now be described with reference to FIGS. 18-20.
The above-described technique involves an image signal processing method which may be performed by a block distortion smoothing apparatus 168 illustrated in FIG. 18. In such smoothing apparatus, an output image signal S160 from an image decoding apparatus, which may be the output image signal S154 from the image decoding apparatus 159 of FIG. 17, is received by a blocking circuit 160. Such output image signal S160 includes a block distortion due to DCT processing in a manner as previously described. The blocking circuit 160 is adapted to process the received image signal having a predetermined number of pixels so as to form a blocked image signal S161 having a larger predetermined number of pixels. For example, if the coded block in the blocking circuit 160 has 8.times.8 pixels, such as a coded block 96 as shown in FIG. 19, the blocked image signal S161 supplied from the blocking circuit 160 represents a so-called processing block and may have 24.times.24 pixels, such as a processing block 98 shown in FIG. 19. (In FIG. 19, the coded blocks 96 are identified by broken lines, and the processing block 98 is identified by a solid line.)
The blocked image signal S161 is supplied to a two-dimensional DCT circuit 161, which is adapted for processing a signal having 24.times.24 pixels, whereupon such received signal is transformed into a blocked coefficient signal S162 having 24.times.24 coefficients. The blocked coefficient signal S162 is supplied to a higher-order coefficient processing circuit 162 which is adapted to set higher-order side coefficients to a value of zero. More specifically, since a distortion observed as a discontinuous line of a block boundary tends to be transformed to a higher-order side coefficient(s), the higher-order side of the transformed coefficient(s) are set to zero as shown in FIG. 20. That is, a shaded portion 97 of FIG. 20 represents the higher-order coefficient(s) of the 24.times.24 vertical and horizontal coefficients which are set to zero. Alternatively, instead of setting the higher-order side coefficient(s) to zero, the values of such higher-order coefficient(s) may be compressed.
A processed coefficient signal S163 from the higher-order coefficient processing circuit 162 is supplied to a two-dimensional inverse DCT circuit 163 which is adapted to perform an inverse DCT operation on the received signal so as to form a processed blocked image signal S164 having 24.times.24 pixels wherein the block distortion is eliminated. Such processed blocked image signal S164 is supplied to a scanning signal circuit 164 so as to form an output image signal S165 which is substantially identical in form to the image signal S160. The image signal S165 is supplied from the block distortion smoothing apparatus 168.
In the above-described conventional block distortion smoothing apparatus 168, DCT and/or inverse DCT processing is performed utilizing relatively large blocks of data, as previously described. As a result, the amount of calculations associated therewith becomes extremely large. Further, utilizing such apparatus to process a dynamic image increases such calculations and/or the complexity thereof. As is to be appreciated, such large amount of calculations increases the total processing time. As a result, attaining a desired and proper operation while minimizing the fabrication complexity of the block distortion smoothing apparatus 168 may be difficult. In addition, when a still image is processed with a general digital signal processor (DSP) or the like, the processing time may still be relatively long.
Further, in the above-described conventional block distortion smoothing apparatus 168, the higher-order side coefficient(s) may be compressed as previously described. As a result, the high region component(s) of the original image along with the block distortion are compressed. Such compression may adversely affect the resolution, thereby causing the picture quality to be reduced. These adverse affects in the resolution may be minimized by changing the type of processing between selected portions of a respective image. For example, the type of processing can be changed between a portion of the respective image in which the block distortion is easily observable and a portion of such image in which the block distortion is not easily observable. However, changing the type of processing based on the transformation coefficients may not be easily accomplished by the conventional block distortion smoothing apparatus 168. As a result, adaptable processing wherein effective processing changes are performed may not be accomplished by such block distortion smoothing apparatus 168.
Thus, the prior art has failed to provide an image signal processing technique for eliminating or reducing block distortions in a decoded image signal obtained by decoding coded data by utilizing a two-dimensional inverse orthogonal transformation in which the coded data is obtained by coding an image signal by utilizing a two-dimensional orthogonal transformation wherein the amount of calculations are relatively low, the operation and/or fabrication of the associated image signal processing apparatus is relatively easy, and which does not adversely affect the image resolution.